Filters that employ a filter medium, generally in combination with a filtration aid in order to avoid premature blinding of the filter medium and thereby to extend the cycle time between back washing procedures, generally operate on the principle that when the filter is turned off, the filtration aid and dirt collected thereby fall to the bottom of the container. When the filter is started up again, inlet fluid serves to stir up the filtration aid usually together with at least some dirt from the bottom of the filter container so that the filter aid becomes coated on to the filter medium once more.
One type of filter that has proved to be highly successful is that described in my issued South African Patents Nos 93/0761 and 2001/2144. This type of filter utilises a series of generally parallel elongate filter elements that have a plastics injection moulded frame having, in cross-section, a number of longitudinally extending ribs radiating from a central axis and the outer edges of which support the fabric sleeve. The construction of the frame is dictated, to some extent, by the tool design and, as a consequence, the operatively upper end of each frame has heretofore been fitted with a moulded collar encircling the upper end of the frame and having a flange that serves to support the downwardly extending filter element in its operative position relative to a divider plate that supports the filter elements in a generally vertical orientation.
This construction, whilst being substantially successful, has proved to result, in some cases, in the possibility of stresses caused by forces exerted on the filter elements, in particular during start-up, leading to failure of the frame in the region of its upper end, and in particular at the collar. This results from the fact that the upper end is the sole support region of the filter element in use.
In addition, if the fabric bag or sleeve is not held adequately tightly over the upper end of the frame, the fabric of the sleeve can be pushed in, by liquid pressure, to the flow path between the radiating ribs and collar with a consequent decrease in flow from the interior of the relevant filter element and an accompanying decrease in filtration efficiency.